1. Field of the Invention
The present invention relates to a method for cleaning heavy hydrocarbon scale from a shell and tube heat exchanger or other such device incorporated in a equipment in a petroleum refining plant or the like that processes petroleum hydrocarbons as a raw material. The present invention also relates to a piping structure for cleaning the device employed in the aforementioned cleaning method.
The present specification is based on patent applications filed in Japan (Japanese Patent Application No. Hei09-221801, Japanese Patent Application No. Hei10-48301), the content of which is incorporated herein by reference.
2. Description of the Related Art
Highly viscous heavy hydrocarbon-derived foulant (hereinafter, referred to as "scale") containing iron sulfide, catalyst and the like adheres on tubes and the like inside heat exchangers or other devices incorporated into coking, fluid catalytic cracker, or atmospheric or vacuum distillation units which process heavy hydrocarbon-derived raw material in a petroleum refining plant or the like. Scale adhesions on these types of equipment reduce the efficiency of heat transfer and have a negative impact on the overall production efficiency of the plant. As a result, the plant must be shut down regularly for a given period of time in order to clean the equipment.
Cleaning of such equipment has conventionally been carried out using a mechanical cleaning method in which foulant is physically removed, or a chemical cleaning method in which the foulant is chemically removed.
Various mechanical cleaning methods are available, such as a jet cleaning method in which high pressure water is continuously sprayed though a small diameter nozzle, so that the force of the water removes the scale; a blast cleaning method in which an abrasive such as sand, alumina, or steel balls is sprayed at high speed, so that the force of impact removes the scale; a pig cleaning method in which pig is compression relayed by means of air or water inside piping to remove the scale; and a cutting cleaning method in which scale is removed by rotating a cutter or brush.
In the case of chemical cleaning methods, a method is available in which scale is dissolved and removed using chemicals which mainly contain surface active agents.
However, when cleaning heavy hydrocarbon scale, considerable time and expense was incurred in order to remove the highly viscous scale, regardless of which of the aforementioned mechanical cleaning methods was employed. For example, water-jet cleaning was carried out to remove foulant from the inner and outer surfaces of tubes of heat exchanger between the residual oil and the crude oil in the petroleum refining plant's atmospheric distillation unit or a vacuum distillation unit. Water jet cleaning was performed when operations in the plant were suspended for overall servicing, or by suspending operation of only a portion of the equipment and disassembling it during plant operations. Equipment disassembling is the operation of extracting the fluid inside a piece of equipment, removing the shell cover, and pulling out the tube bundle.
Disposing the equipment after disassembling and cleaning it requires large equipment. Moreover, much care must be exercised when inserting the tube bundle. It is also necessary to confirm that flammable substances are below a specific concentration during equipment disassembling. As a result, this operation required much time and expense.
Further, this operation was also problematic in that it presented a danger to workers during movement of heavy equipment, cleaning operations, treatment of waste water, and high pressure jet operations. Also, the plant's operational efficiency fell because of the requirement to suspend operations in a long period of time when disassembling the equipment.
In general, chemical cleaning methods have not been sufficiently effective in dissolving and removing heavy hydrocarbon scale. For this reason, when employing conventional chemicals, it was necessary to maintain the temperature at 80.degree. C. during the cleaning process in order to increase the efficiency of scale removal.
However, aqueous cleaning solvents using conventional chemicals presented an environmental hazard by a treatment of the waste cleaning solvent. For this reason, it was necessary to treat the waste cleaning solvent without the environmental hazard, adding to the costs of the procedure. Moreover, the chemical products themselves were expensive, so that overall costs rose.
In order to resolve the problem of the time required for the cleaning operation, U.S. patent application No. 5,425,814 discloses a cleaning technique for hydrocarbon processing equipment, for example. The technique leaves out a pre-cleaning operation, referred to as "steam out", in which steam is sent inside the system to remove volatile components, especially benzene, to a level below the operational standard. Cleaning time is significantly shortened as a result. Stable cleaning is maintained in this technique by compounding, circulating an aqueous solution comprising of terpenes and surface active agents conventionally employed inside the system, and employing a filter in the circulating loop. However, the technique's efficacy in removing heavy hydrocarbon scale adhered to equipment in a plant employing petroleum-derived hydrocarbon raw material remains problematic.
In particular, in recent years, the duration between regular maintenance has lengthened, requiring cleaning of heat exchangers and the like before the next maintenance period. In this case, plant operations are not suspended, but rather the throughput is reduced and the equipment to be cleaned is isolated from the line. Alternatively, operation of a portion of the devices is halted, and OSM (on-steam maintenance) is performed. However, it is desirable to make the cleaning time as short as possible so that productivity is not reduced